Sabri Lamouchi tore into his Nottingham Forest players after they suffered a first defeat in eight games at Birmingham and lost ground on the Sky Bet Championship's top two.
The Forest boss was unhappy after seeing his side take the lead through Tiago Silva only to miss a penalty just before half-time and squander several other chances in the second period, ultimately losing 2-1.
Most of those fell to Lewis Grabban, whose 41st-minute spot-kick was saved by Lee Camp.
The hosts equalised through Scott Hogan a minute later and Kristian Pedersen went on to make the visitors pay an even higher price with a 74th-minute winner.
"We were so stupid, so naive to give the game when you start the way we started," Lamouchi said.
"I don't want to find excuses. Wind, referee, opponent, Samba Sow (injury), transfer window, club, fans – come on. It's just our fault. We need to accept and stop.
"It was not like my team, that's why I am upset and very disappointed. It's not my team, it's not my players. I am not happy with that. I am very disappointed.
"You start the game in the right way, score a great goal to control the game and the penalty situation changed the game.
"In a small situation you can close the game and in reality we opened the game.
"Had Lewis scored, the game would have been over, absolutely, but this can happen. But you are still winning 1-0, so stay in the game, don't give anything.
"Birmingham came back but before they equalised my feeling was we gave them confidence. Just our fault."
Opposite number Pep Clotet was mystified by referee Jeremy Simpson's decision to award the first-half penalty.
"The only penalty I saw today was a clear handball from Nottingham Forest in their own box," he said. "But I think if they would have scored that penalty the game would have been very difficult for us.
"I haven't got a clue what it was awarded for. For our part it looks like it was a handball, but the only one who touched the ball there was Campy
"That lifted us back into the game and Scott's goal after that put us back even, fully deserved."
Discussing the performance of Hogan, who arrived on loan from Aston Villa this week, Clotet added: "We were very demanding on Scott.
"I think he can build a very good partnership with (Lukas) Jutkiewicz. He is a positive threat to the backline and with more games we are going to learn new ways to find him and make him more dangerous.
"At the same time, I see him very committed to the club, to what we are building to the end of the season and we have got to help him to be the player I have in my mind that he can be."
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